Islet transplantation has been recognized as an effective treatment for type-1 diabetes, but this procedure currently offers insulin independence to only less than 20% of the recipients and requires islets from more than one donor. Even for the recipients who have been transplanted islets from multiple donors, recent studies have indicated that only a small portion of the islets successfully engrafts; not to mention, donor tissue availability is always in shortage.
One strategy to offset the shortage of transplantable islets is an ex vivo culturing method to generate sufficient amounts of functional endocrine tissues from the islet progenitors possibly residing in the donor pancreas. Although a number of methods have been proposed, the ex vivo-generated islet-like tissues tend to produce meager amounts of insulin compared to those produced by native islets, often below 1% of the latter, and/or lack appropriate levels of glucose responsiveness, that is, unable to secrete more insulin in response to increasing concentrations of glucose.
Several studies have indicated the existence of islet progenitors in postnatal pancreas and in vivo regeneration of endocrinal cells by applying external stresses; for examples, administering toxins such as alloxan or streptozotocin, subjecting pancreas to such surgical treatments as pancreatectomy or tying a pancreatic duct called duct ligation. Recently, the duct ligation was found to render a massive pancreatic inflammation and cell apoptosis followed by a rapid proliferation of an islet progenitor cell population (Xu et al., 2008, Cell 132:197-207). Although the exact tissue localization of the progenitors in the pancreatic tissues has not yet been elucidated, a number of studies have found evidence that they reside in the ductal or centroacinar regions of the pancreas, as well as within islets indicated by a few other studies.
Despite the lack of exact identity of the progenitors, various methods to isolate them have been tested. One such method is cytometric sorting using a certain set of cell surface markers. A drawback of this method is that the amount of collected cells is often insufficient for expansion culture partly due to the damages caused by the sorting processes and possible biases from the cell surface markers. Another method is the selective expansion of target cells by culturing whole pancreatic discards. The pancreatic discards are seeded in culture containers with appropriate media and growth factors to promote the proliferation of prospective progenitors but not other non-target cells. One challenge in employing this method is to predict which and in what stage of cells need to be selectively expanded with a given culture protocol.
Another strategy to offset the shortage of transplantable islets is an ex vivo expansion of native islet cells. Islet cells such as beta cells are considered terminally differentiated thus difficult to promote their proliferation in vitro, which therefore requires an efficient method to do so.